In commercial inkjet printing systems, a print media is physically transported through the printing system at a high rate of speed. For example, the print media can travel 650 feet per minute. The lineheads in commercial inkjet printing systems typically include multiple nozzle plates, with each nozzle plate having precisely spaced and sized nozzles arranged in a nozzle array. The cross-track pitch, measured as drops per inch or dpi, is determined by the nozzle spacing. The dpi can currently be as high as 600, 900, or 1200 dpi.
A reservoir containing ink or some other material typically is behind each nozzle plate in a linehead. Ink streams through the nozzles in the nozzle plates when the reservoirs are pressurized. The nozzles in the nozzle plates can be very small in size, such as several microns in diameter. Ideally, the nozzles are fabricated to be identical and emit or “jet” parallel streams or drops of ink to produce a uniform density on the print media. But in practice the nozzles are not identical and do not always jet parallel ink drops or streams. Failures in drop deposition can produce artifacts in the content printed on the print media. For example, a blank streak is created when a nozzle stops ejecting ink drops. The blank streak lasts until ink is again ejected from the nozzle.
On the other hand, a “stuck on” jet will produce a dark line for the duration of the “stuck on” event. And the drops ejected from a crooked nozzle frequently intersect with or lie closer to one or more of the neighboring streams to produce a darker streak where the conjoined streams land on the print media and an adjacent lighter streak (or streaks) where the deviated streams are missing from the intended region of the print media.
These artifacts continue until the problem is corrected. Unfortunately, the necessary corrections may not occur for hundreds or thousands of feet of print media, which results in waste when the printed content is not usable. Additionally, wasted print media causes the print job to be more costly and time consuming.
Direct optical inspection of the nozzle plate to determine the straightness of streams from the nozzles is difficult due to the small size of the nozzles. A current method for testing the straightness of streams jetted from the nozzles involves assembling the nozzle plates into a linehead and after the linehead is assembled, testing the nozzle plates to determine if the streams indicate the nozzles are of sufficient quality. This requires a significant amount of time and effort. If one or more streams indicate a nozzle is of inadequate quality, the non-conforming nozzle plate or plates must be removed from the linehead. Removal of the non-conforming nozzle plates further increases the cost of manufacturing of the lineheads. The removal also reduces the manufacturing throughput of lineheads.